1. Field of the Invention
The subject invention relates to an antenna, specifically a planar slot coupling patch antenna, for receiving a circularly polarized radio frequency (RF) signal from a satellite.
2. Description of the Prior Art
Vehicles have long implemented glass to enclose a cabin of the vehicle while still allowing visibility for the driver of the vehicle. Automotive glass is typically either a tempered (or toughened) glass or a laminated glass which is produced by bonding two or more panes of glass together with a plastic interlayer. The interlayer keeps the panes of glass together even when the glass is broken.
Recently, antennas have been integrated with the glass of the vehicle. This integration helps improve the aerodynamic performance of the vehicle as well to help present the vehicle with an aesthetically-pleasing, streamlined appearance. Integration of antennas for receiving linearly polarized RF signals, such as those generated by AM/FM terrestrial broadcast stations, has been the principal focus of the industry. However, that focus is shifting to integrating antennas for receiving RF signals from Satellite Digital Audio Radio Service (SDARS) providers. SDARS providers use satellites to broadcast RF signals, particularly circularly polarized RF signals, back to Earth. SDARS providers use multiple satellites in a geostationary orbit or in an inclined elliptical constellation.
Various antennas for receiving circularly polarized RF signals are well known in the art. Examples of such antennas are disclosed in the U.S. Pat. No. 5,633,645 (the '645 patent) to Day and U.S. Pat. No. 6,778,144 (the '144 patent) to Anderson.
The '645 patent discloses an antenna including a radiation element disposed on a pane of glass. The pane of glass is suitable for application as a window of a vehicle. A ground plane is disposed substantially parallel to and spaced from the radiation element. The ground plane defines a slot having a first leg and a second leg generally perpendicular to each other and forming a cross shape. The radiation element and the ground plane sandwich a dielectric layer. A feed line is disposed on a circuit board attached to the ground plane, such that the feed line is isolated from the ground plane. The feed line traverses a center point of the slot. The antenna of the '645 patent occupies a relatively large area on the pane of glass, which obstructs the view of a driver of the vehicle.
The '144 patent discloses an antenna including a radiation element. The radiation element defines a slot including a first leg and a second leg generally perpendicular to each other and forming a cross shape. The first and second legs are of unequal lengths and/or widths to give the antenna a circular polarization. A ground plane is disposed substantially parallel to and spaced from the first conductive layer. The radiation element and the ground plane sandwich at least one dielectric layer. A plurality of vias electrically connect the first conductive layer to the second conductive layer. A feed line is disposed within the at least one dielectric layer and is substantially parallel to the conductive layers. The feed line is disposed at a 45° angle in relation to the legs of the slot and traverses a center of the cross shape. The antenna of the '144 patent is not integrated with a window of a vehicle.
The characteristics of glass, particularly soda-lime-silica automotive glass, and the angled disposition of this glass when applied as a window of a vehicle, provide challenges to the effective integration of an antenna with a window of the vehicle. Automotive manufacturers demand strict requirements as to the amount of visual obstruction caused by antennas integrated with windows of the vehicle. To date, the performance of antennas integrated with automotive glass in receiving SDARS signals has been disappointing. Therefore, there remains an opportunity to introduce an antenna that aids in the reception of the circularly polarized RF signal from a satellite. Particularly, there remains an opportunity for a high-performing antenna that, when integrated with an automotive window does not create a substantial visual obstruction and still maintains optimal reception.